Recently, as a digital wireless intercom system that supplements drawback of an analog wireless intercom system using a very high frequency (VHF) band and a ultra high frequency (UHF) band, a digital wireless intercom system using a scientific and medical (ISM) band of about 900 MHz/2.4 GHz and an unlicensed national information infrastructure (UNII) band of about 5 GHz that are useful without a license has been widely developed.
In addition, as a standard of a wireless local area network (WLAN) next to an existing IEEE 802.11n WLAN, standardization of IEEE 802.11ac and IEEE 802.11ad that support a multiple access and a speed over 1 Gbps and have a tightened power saving specification for portable device has been actively performed.
The digital wireless intercom system has several advantages. For example, an audio data of high sound quality may be transmitted with a high data transmission speed. In addition, audio channels that can be simultaneously used by a method such as a time division method may be obtained. Further, various value added service may be provided.
Regardless of the advantages, the digital wireless intercom system has a disadvantage such that a distance of wireless communication is shorter than that of the analog wireless intercom system using the VHF band and the UHF band.
In addition, since the ISM band and the UNII band are useful by everyone without limitation in purposes with various devices using a wireless technology such as a digital enhanced cordless telecommunications (DECT), a WLAN, a Bluetooth and a frequency shift keying (FSK), frequency bands having relatively low interference have different characteristics according to service area and service time. As a result, the digital wireless intercom system using the ISM band and the UNII band has a disadvantage such that an optimum frequency band having relatively low interference can not be always used through an equal frequency according to service area and service time.
For the purpose of overcoming the limitation in a distance of wireless communication of the digital wireless intercom system, a method of expanding a distance of wireless communication by installing antennas at various points has been suggested. The digital wireless intercom system including antennas according to the related art will be illustrated with reference to a drawing.
FIG. 1 is a view showing a digital wireless intercom system according to the related art.
In FIG. 1, a digital wireless intercom system 10 according to the related art includes a base station (BS) 20, an antenna splitter 30, first and second active antennas 40 and 42, a plurality of first belt packs 50, a plurality of second belt packs 52 and a plurality of third belt packs 54.
The antenna splitter 30 is connected to the base station 20 wiredly, and the first and second active antennas are connected to the antenna splitter 30 wiredly.
The plurality of first belt packs 50 are located within a first transmission radius R1 that is a distance of wireless communication of the base station 20. In addition, the plurality of second belt packs 52 are located within a second transmission radius R2 that is a distance of wireless communication of the first active antenna 40, and the plurality of third belt packs 54 are located within a third transmission radius R3 that is a distance of wireless communication of the second active antenna 42.
As a result, while the plurality of first belt packs 50 within the first transmission radius R1 of the base station 20 transmits and receives an audio data with the base station 20 wirelessly, the plurality of second belt packs 52 and the plurality of third belt packs 54 outside the first transmission radius R1 of the base station 20 can not directly transmit and receive the audio data with the base station wirelessly.
In this case, although the plurality of second belt packs 52 and the plurality of third belt packs 54 are located at a shadowed area of the base station 20, the plurality of second belt packs 52, the plurality of third belt packs 54 and the base station 20 transmit and receive the audio data with each other wirelessly by expanding the distance of wireless communication of the base station 20 using the first and second active antennas 40 and 42 connected to the base station 20 wiredly through the antenna splitter 30.
In the digital wireless intercom system 10 according to the related art, accordingly, the limitation in a distance of wireless communication may be improved to a certain extent by eliminating the shadowed area of the base station 20 using the first and second active antennas 40 and 42.
However, although an unshielded twisted pair (UTP) cable may be used for a connection line between the base station 20 and the antenna splitter 30 and a connection line between the antenna splitter 30 and the first and second active antennas 40 and 42, a quality of wireless communication may be deteriorated by a length of the connection lines to cause a phenomenon such as a sound disconnection due to delay. In addition, an Ethernet (, which is referred to as a local area network (LAN)) and an infrastructure that are already installed in a building may not be used due to different protocols.
Further, the additional antenna splitter 30 and the additional first and second active antennas 40 and 42 are required, and the additional antenna splitter 30 and the first and second active antennas 40 and 42 are required to be connected through the additional UTP cables.
Since additional equipments are required to expand the distance of wireless communication, installation cost and maintenance cost increase and utilization and generalization have their limitations.
Moreover, an additional signal converting unit is required even when the digital wireless intercom system 10 according to the related art is connected to an internet.
Although the digital wireless intercom system according to the related art has advantages such as the high sound quality due to the high data transmission speed and the multiple audio channels due to the time division method, the digital wireless intercom system according to the related art has disadvantages such as the short distance of wireless communication and the frequency interference. As a result, the digital wireless intercom system according to the related art is restrictively applied to a broadcasting system of high price, and general application of the digital wireless intercom system according to the related art to a building such as a restaurant has a serious obstacle.